1. Field of the Invention
The present invention is directed to a gas discharge lamp and particularly a flash tube which has a glass tube and at least two electrodes which are connected in a gas tight fashion by an intermediate glass at the ends of the glass tube.
2. Prior Art
A gas discharge lamp which is used for a flash tube is disclosed in an article by C. Meyer, "Recent Developments In Electronic-Flash Lamps", Philips Technical Review, Vol. 22, 1960/61, No. 12, pages 377-390. The flash tube such as disclosed in this article in the simplest circumstances may consist of a straight piece of glass tube which has an electrode fused in a gas tight fashion at each end so that an anode is disposed adjacent one end and cathode is at the other end. Generally, the anode consists of tungsten or molybdenum and the cathode consists of a sintered body which comprises saturating substances that are composed of emission materials and getter materials which are well known and described, for example, in German printed patent AS No. 23 32 588. The discharge tube or lamp is filled with an inert gas preferably xenon on account of its spectral light distribution, which is similar to natural daylight. An ignition or triggering electrode is generally located on the outside of the tube.
To initiate the gas discharge, the ignition or triggering electrode initiates the gas discharge between itself and the cathode by producing an electrical field which raises as rapidly as possible and, therefore, the gas adjacent the cathode becomes ionized due to the effects of the field and causes a gas discharge to take place. This gas discharge will extend in the direction of the anode until the field strength of the electrical field prevailing between the cathode and the anode becomes of such a magnitude due to displacement of the part of the gas which has not become ionized that the remaining gas is also ionized. Consequently, the main gas discharge between the cathode and anode is triggered. Initiation of the gas discharge can also take place without a separate ignition or triggering electrode if a so-called "overhead ignition" occurs in which the anode receives an adequate voltage pulse.
The glass tube which serves as a discharge vessel consists of quartz crystal glass or hard glass having a very high melting point. The electrode material or at least the material of the metal or metallic electrodes may be lines or connector pins, which pass through a gas tight seal of the glass tube and extends to the actual electrode arranged inside the glass tube, must be selected to be such that the different coefficients of thermal expansion between the material of the electrical connector pin and the glass tube do not lead to cracks in the gas tight connection or seal. When hard glass is used for the glass tube, this matching can be effected by selecting tungsten for the electrodes or at least for the portion of the electrical connector pin extending through the glass envelope and by matching the coefficient of thermal expansion of the tungsten with a hard glass of appropriate composition. It should be noted that matched glass of this type is commercially available.
In the case of a quartz crystal glass, a direct matching is not possible. In this instance, as in the case when a hard glass is used in fact for the glass tube, but for economic reasons and primarily to reduce cost, nickel is used as the connector pin instead of the more expensive tungsten, a transition element composed of an intermediate glass must be provided in order to match different coefficients of thermal expansion.
Although tungsten in combination with a matched hard glass has an advantage in comparison with other metals that no intermediate glass is required, the cost of tungsten is relatively high and tungsten cannot be soldered. A compromise of using expensive metal, which can sustain a high thermal load, only for the actual electrodes, of employing a sintered body for the cathode, and of producing the electrical connector pins for the two electrodes from a cheap metal necessitates utilizing an intermediate glass, which results in an equally expensive solution due to the high cost of the process steps which are required.